Right, after TOS, they started saying that there had been an apocalyptic war. But in the ORIGINAL series, they had avoided that one, although they had had plenty of smaller ones, that didn't threaten the whole planet like the ones they talk about in TNG and later.

And the rule with ANY Star Trek is, no harshing on the plot! (That means no pointing out the gaping holes. Just wait for the next one to come along and either plug them, or give you new ones to harsh on {but not while watching, only while rehashing!})